1. Field of the Invention
The present invention relates to a fuel cell system including a fuel cell, a fuel gas supply apparatus for supplying a fuel gas to a fuel gas flow field, an oxygen-containing gas supply apparatus for supplying an oxygen-containing gas to an oxygen-containing gas flow field, a scavenging gas supply apparatus for supplying air as a scavenging gas to the fuel gas flow field, and a control device. Further, the present invention relates to a scavenging method for such a fuel cell system.
2. Description of the Related Art
The fuel cell is a system for obtaining direct current electrical energy by electrochemical reactions of a fuel gas (chiefly hydrogen-containing gas) supplied to an anode and an oxygen-containing gas supplied to a cathode.
A solid polymer electrolyte fuel cell includes a power generation cell formed by sandwiching a membrane electrode assembly between separators. The membrane electrode assembly includes the anode, and the cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a solid polymer ion exchange membrane. In use of this type of power generation cell, generally, predetermined numbers of the membrane electrode assemblies and separators are alternately stacked together to form a fuel cell stack.
In this type of fuel cell, during electric generation, water is produced at the cathode, and the water is retained at the anode, due to back diffusion from the cathode. Consequently, when electric generation is halted, the residual water needs to be discharged from the cathode and the anode.
In this regard, for example, a fuel cell system disclosed in Japanese Laid-Open Patent Publication No. 2006-086015 includes a fuel cell that generates electricity by causing an electrochemical reaction between a fuel gas and an oxidant gas (an oxygen-containing gas), a fuel gas supply system that supplies the fuel gas to a fuel electrode of the fuel cell, an oxidant gas supply system that supplies the oxidant gas to an oxidant electrode of the fuel cell, humidification means for humidifying the oxidant gas, and communication means for switchably connecting and disconnecting between the fuel gas supply system and the oxidant gas supply system in an open/closed manner. At system shutdown, supply of the fuel gas to the fuel electrode of the fuel cell is stopped and at the same time, an oxidant gas that is drier than the oxidant gas supplied to the oxidant electrode for normal electricity generation is supplied to the oxidant electrode, and thereafter, the drier oxidant gas is also supplied to the fuel electrode through the communication means, and finally the fuel cell system is shut down.
Thus, the above fuel cell system first stops supplying the fuel gas to the fuel electrode of the fuel cell at system shutdown and at the same time starts supplying a dry oxidant gas such as air to the oxidant electrode, so as to reduce the water content in the catalyst layer of the oxidant electrode. The fuel cell system then stops operations after supplying the oxidant gas such as air also to the fuel electrode to replace the fuel gas in the fuel electrode with the oxidant gas, so that deterioration caused at system shutdown as well as during storage can be suppressed.
However, according to the fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-086015, since scavenging of the anode (fuel electrode) by air is performed every time operation of the fuel cell is stopped, scavenging is performed frequently, and the anode is thus subject to deterioration. Also, when the fuel cell is started after the air scavenging, the anode is damaged at start operation. Further, when the anode is scavenged by air, large noises from an air compressor or the like are produced, and energy efficiency becomes low.